This invention relates to a die bonding method for adhering semiconductor chips to a lead frame.
When assembling a semiconductor device, the die pad surface of a lead frame is coated with an adhesive paste such as silver epoxy paste, and a semiconductor chip called a die is pressed thereon, to adhere the die to the die pad. At this time, if too much adhesive paste is applied, it may flow out and cause problems in the subsequent wire bonding work. On the other hand, if the application of adhesive paste is insufficient, the air remaining in the adhesive paste is not forced out completely from the layer of the adhesive paste when spreading the adhesive paste with the die, and voids may be left in the adhesive paste. When the adhesive paste is cured in such a state, not only is the adhesive strength of the die lowered, but the thermal and electrical conductivity between the die and die pad is also lowered.
To solve such problems, hitherto, the die bonding method as shown in FIGS. 12 to 17 has been known.
First, as shown in FIG. 12, plural nozzles 21 are disposed at the end of a nozzle head, and an adhesive paste 24 is applied in spots on the surface of a die pad 23 of a lead frame 22 from each nozzle 21. Then, as shown in FIG. 13, while holding a die 26 by means of a chuck 25, the back side of the die 26 is pressed against the surface of the die pad 23, and the position of the die 26 is determined while spreading the adhesive paste 24 outward, and the adhesive paste 24 is cured in the state shown in FIG. 14.
In this die bonding method, however, the same problems as mentioned above occur. That is, in the conventional die bonding method shown in FIGS. 12 to 14, plural nozzles are arranged in a matrix as shown in FIG. 15. Accordingly, when the adhesive paste 24 is applied to the surface of die-pad 23 and the die 26 is pressed, the air remaining between the plural spots of adhesive page 24 and the die 26 is often not expelled out completely to the outside through the spots of the adhesive page 24. Voids 27 are then left over in the adhesive paste 24 as shown in FIG. 16. When the adhesive paste 24 is cured in this state, as shown in FIG. 17, voids 27 remain in the adhesive paste 24, and the adhesion strength of the die 26 is weak, and the die 26 may be easily separated from the die pad 23, or the thermal and electrical conductivity from the die 26 to the die pad 23 may be worsened.